1. Field of Invention
Embodiments relate to electronic amplifier output signal power detection and control, particularly for radio frequency (RF) signals output from one or more final stage amplifying transistors.
2. Related Art
Many electronic circuit applications require that output power of an amplifier be detected and controlled to maintain a specified level. Such an output power level may be required to change with time. In a Global System for Mobile communications (GSM) cellular telephone handset, for example, the final stage radio frequency (RF) amplifier outputs a signal to the antenna, and the power level of this signal must be controlled to maintain one or more precise levels. Therefore, it is important to be able to determine amplifier output power level and to control that power level.
The power of a signal output from a Class C or similar transistor amplifier with non-continuous conduction corresponds to the collector current. In such amplifiers, two or more transistors may be connected in parallel so that the signal output from the transistors corresponds to the combined collector currents. As is well-known for a bipolar junction transistor that operates in the cutoff or forward active region, the base current controls the collector current, and the collector current depends on the base-emitter voltage. Thus the base-emitter voltage is strongly correlated with the power of the signal output from the amplifier. It is therefore known that a desired output power level of an amplifying transistor can be maintained by sensing the output power and then adjusting the base bias voltage of the transistor to maintain the desired output power level.
One well-known method of sensing the power of an output signal from a transistor amplifier is to use a directional coupler. Such a directional coupler senses either forward or reflected signal power in a transmission line. A disadvantage of directional couplers, however, is that some coupling loss exists as the output signal is sampled. In addition, directional couplers are typically large, a condition which prevents the coupler from being on the same integrated circuit as the amplifier. Thus, additional area is required in an electronic device (e.g., a cellular telephone handset) to accommodate the coupler. Furthermore, adding an off-chip coupler to an electronic device increases the cost of the device.
What is desired is a way of sensing and controlling amplifier output signal power without significant loss, with little expense, and with little use of device area.
In a power sensing and control circuit, the base terminal of a sense transistor is coupled to the base terminal of an amplifying transistor in, for example, the final stage of an RF amplifier. As a result, the sense current in the sense transistor corresponds to the current in the amplifying transistor. The sense current creates a sense voltage at a point along the current path that includes the sense transistor. The AC portion of the sense voltage is shunted to ground. The DC portion of the sense voltage is applied to one input of an error amplifier. A power control voltage is applied to the other input of the error amplifier, and the error amplifier outputs a voltage proportional to the difference (error) between the two input voltages. The error amplifier output is routed back to the base terminal of the amplifying transistor in a negative feedback loop, thereby controlling current in the amplifying transistor. Various combinations of amplifying and sense transistor pairs are included in various embodiments.
The sense transistor, and hence one input of the error amplifier, is coupled to a reference voltage supply. In some embodiments the output of the error amplifier will change as the reference voltage changes. In other embodiments, the output of the error amplifier is made independent of changes in the reference voltage.
Embodiments of the power control circuit offer several advantages, especially if formed on the same integrated circuit chip as the amplifier. In addition to being independent of the reference voltage (e.g., a power supply voltage) supplied to the chip, the power control circuit takes up only a small area on the chip. Further, cost is minimal since the power control circuit may be simultaneously fabricated with the amplifier and any associated on-chip circuits. Further, no output power is lost since the sense signal is taken from the input side of the amplifier. Such advantages are useful in, for example, cellular telephone handsets.